1. Field of the Invention
The present invention relates to protective granules for asphalt roofing shingles, roofing shingles employing such granules, and processes for making such granules and shingles.
2. Brief Description of the Prior Art
Sized mineral rocks are commonly used as granules in roofing applications to provide protective functions to the asphalt shingles. Roofing granules are generally used in asphalt shingles or in roofing membranes to protect asphalt from harmful ultraviolet radiation. Roofing granules typically comprise crushed and screened mineral materials, which can be coated subsequently with a binder containing one or more coloring pigments, such as suitable metal oxides. The granules are employed to provide a protective layer on asphaltic roofing materials such as shingles, and to add aesthetic values to a roof.
Depending on location and climate, shingled roofs can experience very challenging environmental conditions, which tend to reduce the effective service life of such roofs. One significant environmental stress is the elevated temperature to which roofing shingles are subjected under sunny, summer conditions.
Mineral-surfaced asphalt shingles, such as those described in ASTM D0225 or D03462, are generally used in steep-sloped roofs to enhance the water-shedding function while adding aesthetically pleasing appearance to the roofs. The asphalt shingles are generally constructed from asphalt-saturated roofing felts and surfaced by pigmented color granules, such as those described in U.S. Pat. No. 4,717,614. Asphalt shingles coated with conventional roofing granules are known to have low solar heat reflectance, and hence will absorb solar heat, especially through the near infrared range (700 nm-2500 nm) of the solar spectrum. This phenomenon is increased as the granules covering the surface become dark in color. For example, while white-colored asphalt shingles can have solar reflectance in the range of 25-35%, dark-colored asphalt shingles can have solar reflectance of only 5-15%. Furthermore, except in the white or very light colors, there is typically only a very small amount of pigment in the color coating of the conventional granules that reflects solar radiation well. As a result, it is common to measure temperatures as high as 77° C. on the surface of black roofing shingles on a sunny day with 21° C. ambient temperature. Absorption of solar heat may result in elevated temperatures at the shingle's surroundings, which can contribute to the so-called “urban heat-island effect” and increase the cooling load to its surroundings. This not only increases the demand for indoor cooling energy, but also contributes to smog formation due to higher surface temperatures. Hence, it is beneficial to have a surface with increased solar reflectance, such as greater than 70 percent, to reduce solar heat gain, thereby reducing the heat flux entering the building envelope or reducing surface temperatures for lowering smog formation. It is therefore advantageous to have roofing shingles that have high solar reflectance.
The surface reflectivity of an asphalt shingle largely depends on the solar reflectance of the granules that are used to cover the bitumen. Typically, roofing granules are applied such that about 95 to 97 percent of the shingle surface is effectively covered by the granules.
The state of California has implemented a building code requiring the low-sloped roofs to have roof coverings with solar reflectance greater than 70%. However, colored roofing granules, prepared using current coloring technology, are not generally capable of achieving such a high level of solar reflectance. Thus, in order to reduce solar heat absorption, it has been suggested to apply coatings externally directly onto the shingled surface of roofs. White pigment-containing latex coatings have been proposed and evaluated by various manufacturers. However, the polymeric coating applied has only limited amount of service life and requires re-coat after certain years of service. Also, the cost of adding such a coating on roof coverings can be relatively high. Other manufactures have also proposed the use of exterior-grade coatings that were colored by IR-reflective pigments for deep-tone colors and sprayed onto the roof in the field. U.S. Pat. No. 7,238,408 discloses an asphalt-based roofing material comprising a mat saturated with asphalt coating and a top coating having a top surface layer that has a solar reflectance of at least 70% obtained by adding metal flakes or metal flakes coated with metal oxides or sulfides such as titanium dioxide or zinc sulfide. U.S. Pat. No. 6,933,007 discloses the use of novel granules with solar reflectance greater than 55% and hardness greater than 4 on Moh's scale to enhance the solar reflectivity of asphalt based roofing products.
Solar control films that contain either a thin layer of metal/metal oxides, or dielectric layers applied through vacuum deposition, have been commercially available for use in architectural glasses.
U.S. Pat. No. 7,273,634 discloses pulverizing recycled ceramic material such as furnace liner blocks to form a powder and granular ceramic material to be dispersed in an adhesive and used for coating roofing shingles.
Many materials have been proposed for use in protecting roofing from solar heat radiation, such as ceramic grog, recycled porcelain (U.S. Pat. No. 6,933,007, incorporated herein by reference), and white plastic chips. However, the previously proposed materials have limited use, and cannot satisfy all requirements for roofing materials.
There is a continuing need for roofing materials, and especially asphalt shingles, that have improved resistance to thermal stresses while providing an attractive appearance. In particular, there is a need for roofing granules that provide increased solar heat reflectance to reduce the solar absorption of the shingle. Hence, it would be advantageous to have a roofing product that has solar reflectance greater than 70%. It is further advantageous that such a product could be applied to other substrates, such as siding or paving, or can be constructed as part of the system exposed to solar radiation such that the solar heat gain can be reduced to mitigate the urban heat island effect.